Prothena Corporation plc Schedule 13G/A shows Rubric Capital Management and related reporting person David Rosen hold 5,000,000 Ordinary Shares, representing 9.29% of the class. The filing states the share count basis is 53,832,982 Ordinary Shares outstanding as of March 2, 2026. The cover page reports the position is held with shared voting and shared dispositive power by the reporting persons. The statement identifies Rubric Capital Master Fund LP as a Rubric Fund with rights to dividends or sale proceeds on more than 5% of the class.
Positive
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Negative
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Insights
Schedule 13G/A reports a passive >5% stake held with shared voting/dispositive power.
The filing discloses that 5,000,000 Ordinary Shares are attributable to Rubric Capital and David Rosen, equal to 9.29% of the outstanding shares reported as of March 2, 2026. The filing uses shared voting/dispositive power language rather than sole control.
Cash‑flow treatment and any planned transactions are not detailed in the excerpt; subsequent filings would show changes in holdings or voting intentions if they occur.
Key Figures
Reported shares owned:5,000,000 sharesPercent of class:9.29%Shares outstanding:53,832,982 Ordinary Shares+2 more
5 metrics
Reported shares owned5,000,000 sharesAmount attributed to Rubric Capital and David Rosen
Percent of class9.29%Calculated based on outstanding shares as of March 2, 2026
Shares outstanding53,832,982 Ordinary SharesOutstanding as of <date>March 2, 2026</date> per issuer proxy statement
Shared voting power5,000,000 sharesShared voting power reported on cover page
Shared dispositive power5,000,000 sharesShared dispositive power reported on cover page
Key Terms
Schedule 13G/A, shared dispositive power, beneficially owned, Ordinary Shares, par value $0.01
4 terms
Schedule 13G/Aregulatory
"This statement is filed by: (i) Rubric Capital Management LP ..."
A Schedule 13G/A is an amended public filing with the U.S. securities regulator that updates a previous Schedule 13G, disclosing when an individual or group holds a substantial (typically over 5%) stake in a company and is claiming a passive, non‑controlling intent. Investors monitor these updates because rising or falling holdings can signal changing confidence, potential future moves, or shifts in voting power — like watching a public ledger where large shareholders quietly adjust their positions.
shared dispositive powerregulatory
"Shared Dispositive Power 5,000,000.00"
beneficially ownedfinancial
"Amount beneficially owned: The information required by Items 4(a) - (c) is set forth"
Beneficially owned describes securities or assets where a person has the economic rights and control—such as the right to receive dividends and to direct voting—even if legal title is held in another name. Think of it like having the keys and using a car that’s registered to someone else: you get the benefits and make decisions. Investors care because beneficial ownership reveals who truly controls value and voting power, affecting corporate decisions and takeover dynamics.
Ordinary Shares, par value $0.01financial
"Title of class of securities: Ordinary Shares, par value $0.01 per share"
What stake does Rubric Capital hold in Prothena (PRTA)?
Rubric Capital and David Rosen are reported as owning 5,000,000 Ordinary Shares, equal to 9.29% of the class. This percent is calculated using 53,832,982 Ordinary Shares outstanding as of March 2, 2026.
Does the Schedule 13G/A show who controls the shares?
The filing states the shares are held with shared voting power and shared dispositive power. It does not assert sole voting or sole dispositive authority for Rubric Capital or David Rosen.
Which Rubric entity is identified as having dividend or sale proceeds rights?
The filing identifies Rubric Capital Master Fund LP, a Rubric Fund, as having the right to receive dividends or proceeds from sale of more than 5% of the Ordinary Shares.
What outstanding share count is used to calculate the 9.29% stake?
The percent ownership is based on 53,832,982 Ordinary Shares outstanding as of March 2, 2026, as reported in the issuer’s proxy statement filed March 27, 2026.
Who signed the Schedule 13G/A for the reporting persons?
The filing is signed by Michael Nachmani as Chief Operating Officer and by David Rosen individually, both dated May 15, 2026.
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 13G
UNDER THE SECURITIES EXCHANGE ACT OF 1934
(Amendment No. 1)
PROTHENA CORPORATION PUBLIC LIMITED COMPANY
(Name of Issuer)
Ordinary Shares, par value $0.01 per share
(Title of Class of Securities)
G72800108
(CUSIP Number)
03/31/2026
(Date of Event Which Requires Filing of this Statement)
Check the appropriate box to designate the rule pursuant to which this Schedule is filed:
Rule 13d-1(b)
Rule 13d-1(c)
Rule 13d-1(d)
schemaVersion:
SCHEDULE 13G
CUSIP Number(s):
G72800108
1
Names of Reporting Persons
Rubric Capital Management LP
2
Check the appropriate box if a member of a Group (see instructions)
(a)
(b)
3
Sec Use Only
4
Citizenship or Place of Organization
DELAWARE
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
5,000,000.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
5,000,000.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
5,000,000.00
10
Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)
11
Percent of class represented by amount in row (9)
9.29 %
12
Type of Reporting Person (See Instructions)
IA, PN
SCHEDULE 13G
CUSIP Number(s):
G72800108
1
Names of Reporting Persons
David Rosen
2
Check the appropriate box if a member of a Group (see instructions)
(a)
(b)
3
Sec Use Only
4
Citizenship or Place of Organization
UNITED STATES
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
5,000,000.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
5,000,000.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
5,000,000.00
10
Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)
11
Percent of class represented by amount in row (9)
9.29 %
12
Type of Reporting Person (See Instructions)
HC, IN
SCHEDULE 13G
Item 1.
(a)
Name of issuer:
PROTHENA CORPORATION PUBLIC LIMITED COMPANY
(b)
Address of issuer's principal executive offices:
77 Sir John Rogerson's Quay, Block C Grand Canal Docklands Dublin 2, L2 D02 VK60
Item 2.
(a)
Name of person filing:
This statement is filed by:
(i) Rubric Capital Management LP ("Rubric Capital"), the investment adviser to certain investment funds and/or accounts (collectively, the "Rubric Funds") that hold the Ordinary Shares, par value $0.01 per share (the "Ordinary Shares") of Prothena Corporation plc, an Irish public limited company (the "Issuer") reported herein; and
(ii) David Rosen ("Mr. Rosen"), Managing Member of Rubric Capital Management GP LLC, the general partner of Rubric Capital.
The foregoing persons are hereinafter sometimes collectively referred to as the "Reporting Persons."
The filing of this statement should not be construed as an admission that any of the forgoing persons or any Reporting Person is, for the purposes of Section 13 of the Act, the beneficial owner of the Ordinary Shares reported herein.
(b)
Address or principal business office or, if none, residence:
The address of the principal business office of each of the Reporting Persons is 155 East 44th St, Suite 1630, New York, NY 10017.
(c)
Citizenship:
Rubric Capital is a Delaware limited partnership. Mr. Rosen is a citizen of the United States of America.
(d)
Title of class of securities:
Ordinary Shares, par value $0.01 per share
(e)
CUSIP No.:
G72800108
Item 3.
If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
(a)
Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)
Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)
Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)
Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)
An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)
An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)
A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)
A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)
A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)
A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J),
please specify the type of institution:
(k)
Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.
Ownership
(a)
Amount beneficially owned:
The information required by Items 4(a) - (c) is set forth in Rows 9 of the cover page for each of the Reporting Persons and is incorporated herein by reference.
The percentage set forth in Row (11) of the cover page for each of the Reporting Persons and in Item 4(b) is based on the 53,832,982 Ordinary Shares outstanding as of March 2, 2026, as reported in the Issuer's Definitive Proxy Statement on Schedule 14A filed with the Securities and Exchange Commission on March 27, 2026.
(b)
Percent of class:
9.29 %
(c)
Number of shares as to which the person has:
(i) Sole power to vote or to direct the vote:
The information required by Item 4(c)(i) is set forth in Row 5 of the cover page for each of the Reporting Persons and is incorporated herein by reference.
(ii) Shared power to vote or to direct the vote:
The information required by Item 4(c)(ii) is set forth in Row 6 of the cover page for each of the Reporting Persons and is incorporated herein by reference.
(iii) Sole power to dispose or to direct the disposition of:
The information required by Item 4(c)(iii) is set forth in Row 7 of the cover page for each of the Reporting Persons and is incorporated herein by reference.
(iv) Shared power to dispose or to direct the disposition of:
The information required by Item 4(c)(iv) is set forth in Row 8 of the cover page for each of the Reporting Persons and is incorporated herein by reference.
Item 5.
Ownership of 5 Percent or Less of a Class.
Not Applicable
Item 6.
Ownership of more than 5 Percent on Behalf of Another Person.
If any other person is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, such securities, a statement to that effect should be included in response to this item and, if such interest relates to more than 5 percent of the class, such person should be identified. A listing of the shareholders of an investment company registered under the Investment Company Act of 1940 or the beneficiaries of employee benefit plan, pension fund or endowment fund is not required.
See Item 2(a). Rubric Capital Master Fund LP, a Rubric Fund, has the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, more than 5% of the Ordinary Shares.
Item 7.
Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
Not Applicable
Item 8.
Identification and Classification of Members of the Group.
Not Applicable
Item 9.
Notice of Dissolution of Group.
Not Applicable
Item 10.
Certifications:
By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under ?? 240.14a-11.
SIGNATURE
After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.
